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It was early on a Saturday evening in San Francisco, and I had just ordered a glass of wine at Vinyl, a new wine bar in the NoPa district, north of the Haight. The place was relatively emptya bartender behind the bar, three people with at least three piercings apiece at a little table across the way, plus Mark Bright, Vinyl's co-owner. I was sitting on a couch in the back, where a Clint Eastwood spaghetti Western played on a screen tacked to the wall. In front of me was an antique record player that looked like it had been bought at a flea market; it was now being used as a table.

When you order a glass of wine at Vinyl, you get a choice. You can pick from a list of about 34 bottles, the usual way. Or you can opt to design one of your own wine blends, using a combination of wines that Bright makes in Sonoma. On this day, the options were a Sangiovese and a Montepulciano, the two red varieties that are traditionally combined in the rustic wines of Italy's Abruzzo region.

I took a sip of each. The Sangiovese was lightly tannic, with bright red-cherry notes and fairly zingy acidity. The Montepulciano was fleshier, a lot more tannic and overall more rough-and-tumble; sort of a workman's wine. I poured 150 milliliters of Sangiovese into the beaker, added 50 milliliters of Montepulciano, swirled it around, and poured a sip or so into my glass. Not bad. Not great, but not bad. A little ragged around the edges. I added another 50 milliliters of Sangiovese and gave it another sip. Getting there, I thought. Definitely getting there.

As Americans, we tend to think of wines in terms of grape varietiesspecifically single-grape varieties. When we buy a bottle at the supermarket, it's usually a Chardonnay, a Cabernet Sauvignon, a Merlot. But a huge percentage of the 35 billion or so bottles produced each year in the world are wine blends of multiple grape varieties. When you drink a Côtes-du-Rhône, or a red Bordeaux, or a basic brut Champagne, it's a blend.

What's more, many of the wines we think are made from a single type of grape actually aren't. Federal law, for instance, requires that a wine contain only 75 percent of the grape variety on the label. That three-liter box of Chardonnay you just bought may be (to use the 2009 Black Box Monterey County Chardonnay as an example) only 86 percent Chardonnay. The remaining 14 percent in Black Box is a bizarre grab bag of White Riesling, Gewürztraminer, Malvasia Bianca, Muscat Canelli and Pinot Gris (an extreme case, but a surprisingly pleasant one, too).

Blending can be an extraordinarily useful winemaking technique. By combining grape varieties, winemakers can accentuate a wine's virtues or diminish its weak points; they can add a touch of spice to the aroma, say, or a bit more body to the texture. Cabernet Sauvignon is one of the world's great red grapes, but it can be extremely tannic when young. A little Merlotas the winemakers of Bordeaux have known for centuriescan round out those harsh tannins, tame Cabernet's innate aggressiveness, make it a bit more forgiving; flip that by adding a bit of Cabernet to a wine that's primarily Merlot, and it's like giving your juice assertiveness training.

Syrah, another variety, is full of black fruit and spice and gamey notes and tannic power. But oddly, if you ferment it with a little of the white grape Viogniersomething discovered years back by winemakers in Côte-Rôtie, in France's northern Rhônethe wine's dark color actually intensifies, and that already intriguing aroma can become even more complex.

None of this is new, of course. To take a classic example: In 1872, the Baron Ricasoli, owner of Tuscany's Brolio estate, is said to have codified the original "recipe" for Chianti. He had worked on it for more than 30 years, experimenting with different grape varieties and percentages, and had finally come up with a formula that required a little more than two-thirds Sangiovese, plus smaller percentages of Canaiolo and the white grape Malvasia. And he had good reasons for these proportions, as he wrote in a letter to a professor at the University of Pisa: "The wine receives most of its aroma from the Sangioveto, as well as a certain vigor in taste; the Canajuolo gives it a sweetness which tempers the harshness of the former without taking away any of its aroma, though it has an aroma all of its own; the Malvagia, which could probably be omitted for wines for laying down, tends to dilute the wine made from the first two grapes, but increases the taste and makes the wine lighter and more readily suitable for daily consumption."

What is new is a recent boom in affordable, branded, blended American wines. Walk down the aisles at a liquor superstore like California's BevMo and you'll see dozens of these wine blends, typically with quirky names and price tags under $20: Hot to Trot Red, Hey Mambo Swanky White, Whistle Stop Red, Unruly Red, you name it. They are popularsales were up eight percent in the first six months of this year. They're also useful for wineries; the only thing the wine is known by is a made-up name, so as a winemaker, you don't need to worry too much if you change the blend a little each year. After harvest, if you have a leftover barrel or two of Petite Sirah, no problem. Just chuck it in your brand-new Kitchen Sink Red.

Some wine blends, on the other hand, are far more complex. (They're also typically given far more imposing names, like Dominus, Ovid, Isosceles and Rubicon, which all sound as though they were picked up on the fly at the local random-Latin-words depot.) At California's Joseph Phelps Vineyards, winemaker Ashley Hepworth is responsible for making Insignia, a $200-or-so blend of the Bordeaux varieties Cabernet Sauvignon, Merlot, Malbec and Petit Verdot. Insignia was the first Bordeaux blend produced in Napa Valley to be given a fancy one-word monikerthe first vintage was 1974and it's still one of the most famous. (It's also one of the best.) But putting it together seems like a task not just for a winemaker but a math genius: "When we blend Insignia," Hepworth says, "we start by tasting the Cabernets. We have seven different vineyards we use, and each vineyard produces 10 to 20 different lots of Cabernetwe taste all those. Then we go to the smaller percentages in the blend, the Petit Verdot, Malbec and Merlot, from each vineyard, and all the different lots of those. Then there are choices about barrels: what percentage of heavy versus medium toast, what percentage from each of the five different coopers we use." All of this adds up to some 500 or so possible components. Each year, Hepworth spends the entirety of March and April blending Insignia. For two months, it's her entire focus.

Back at Vinyl, I was still working on my attempt at a backward Montepulciano d'Abruzzo (in Abruzzo, Sangiovese can be no more than 15 percent of the blend; in my wine, it was around 80 percent). My situation was nothing like Hepworth's: I didn't have hundreds of barrels, or dozens of vineyards, or even more than two grape varieties. And no one was going to fire me if I screwed upthough, admittedly, Hepworth doesn't have to work with Clint Eastwood blowing people away in the background, or a table of nose-ringed twentysomethings staring at her.

I took a sip of my final blend, which I'd adjusted to 82 percent Sangiovese and 18 percent Montepulciano. I'd like to say it was brilliant; I will say that it wasn't bad at all. It was in balance. The Baron Ricasoli would not have shunned it. I decided I'd call it Caveat Emptor.

Bright had been noodling with his own blend, so I asked him if I could taste it. He'd opted for substantially more Montepulciano and substantially less Sangiovese. I took a sip and made a face, because the other joy of blending at Vinyl is that it allows you to give your friends a pointlessly hard time.

"Holy Moses, Mark," I said. "Talk about tannic. That stuff's painful!" Of course, the truth was that his blend was just as good as mine. It was simplydespite being made from the same winescompletely different. •